The present invention relates to a small hole electric discharge machine using an elongate pipe electrode having a diameter of 1.0 mm or less for forming a small and deep hole having a depth at least ten times larger than the diameter in a workpiece by generating electric discharge between the pipe electrode and the workpiece. Particularly, it relates to a small hole electric discharge machine for forming an ultrafine and deep hole using an elongate pipe electrode having a diameter of 0.1 mm or less.
Small hole electric discharge machines typically utilize a copper, brass or tungsten elongate pipe electrode tool having a diameter of 1.0 mm or less to form a small hole, i.e., a hole, having a depth at least ten times larger than the diameter. Small hole electric discharge machines include a power supply device for applying a train of power pulses across a working gap formed between the lower end of the pipe electrode and the workpiece, and a fluid supply device for supplying a dielectric fluid, such as water of high resistivity or mineral oil, into the pipe electrode at high pressure. The upper end of the pipe electrode is held by an electrode holder, which is attached to a rotatable spindle in a machine head. The machine head is moveable in the direction of a vertical Z-axis by a Z-axis servo motor. An electrode guide is connected to a W-axis slider, which is moveable in the direction of a vertical W-axis, and is positioned close to the top surface of the workpiece.
During electric discharge machining, the pipe electrode, guided by the electrode guide, is moved gradually downward. By application of a train of power pulses, the insulation characteristics of the dielectric fluid in the work gap is broken down and electric discharges are generated. Microscopic amounts of the workpiece material are molten and blown out due to electric discharges, and are entrained in the dielectric fluid. Microscopic amounts of the pipe electrode material are also removed and become entrained in the dielectric fluid. The amount of wear on the pipe electrode tool is typically the same as or greater than the downward movement of the pipe electrode. The fine fragments removed from the workpiece and the pipe electrode are washed away from the work gap by dielectric fluid which flows through a dielectric fluid exit in the pipe electrode.
The working gap is maintained at a generally constant size. For this purpose, an NC device compares a mean gap voltage detected at the work gap to a reference servo-voltage, and controls the Z-axis servo motor based on the comparison result. In general, the pipe electrode is moved downward when mean gap voltage is higher than the reference servo-voltage, and the pipe electrode is moved, i.e., upward away from the bottom of the machined hole, when mean gap voltage is lower than the reference servo-voltage. Such servo movement may vary from a few xcexcm to a few tens of xcexcm.
A rotating device for rotating the pipe electrode at a few thousand rpm is provided on the machine head. Rotation of the pipe electrode improves roundness of the hole being formed in the workpiece and contributes to removal of the fine fragments from the work gap.
With a small hole electric discharge machine, a jump operation may be performed where an electrode tool is periodically reciprocated rapidly in the Z-axis direction to almost totally expel contaminated dielectric fluid from the hole in the workpiece. Unfortunately, such an operation may cause the elongate pipe electrode to become curved. Further, it is almost impossible to perform a jump operation at all with an elongate pipe electrode having a diameter of 0.1 mm or less. Also, the smaller the outer diameter of the pipe electrode used, the smaller the inner diameter becomes and a smaller quantity of dielectric fluid will be supplied to the work gap. For example, for a pipe electrode having an outer diameter of 0.30 mm, the diameter of the pipe electrode is normally 0.12 mm, 0.07 mm when the outer diameter is 0.15 mm, and 0.04 mm when the outer diameter is 0.1 mm. Thus, if a pipe electrode having a smaller outer diameter is used, electric power to be applied across the work gap will be limited to a relatively small value. The smaller electric power results a in smaller work gap which in turn makes it more difficult to remove fragments from the bottom of the hole being formed. Especially when a deeper hole is being formed in the workpiece, this makes it more difficult to remove fragments at the bottom of the hole.
If a large amount of fragments remain in the working gap, the working gap is likely to be frequently short circuited. When the NC device detects a short-circuit condition at the working gap, the pipe electrode is moved upward away from the machined hole and the power supply device is controlled to maintain an OFF time of power pulse for a sufficient time for a recovery from the short-circuit condition. Not only does the short-circuit current not contribute to the progress of electric discharge machining, it is moreover, likely to damage the pipe electrode. Short-circuit current may result in the pipe electrode being broken off. As such a broken piece is likely to be larger in size than the fine fragments produced by electric discharge machining, it is difficult to remove.
Moreover, if broken pieces accumulate at the bottom of the machined hole, they may cause a short-circuit of the working gap and even close off the opening at the lower end of the pipe electrode. Thus, as the formed hole becomes deeper, it becomes increasingly difficult to continue electric discharge machining.
Recently, it has become possible to form holes having a depth about 50 times larger than the diameter of a pipe electrode using pipe electrode having a diameter of 0.1 mm or less. However, it has been almost impossible to form holes having a depth more than about 50 times larger than the pipe electrode diameter when using a pipe electrode having a diameter of 0.1 mm or less.
An object of the present invention is to provide a small hole electric discharge machine which capable of forming a deeper hole in a workpiece when using an elongate fine pipe electrode having a diameter of even 0.1 mm or less.
Another object of the present invention is to provide a small hole electric discharge machine which can form a hole in the workpiece using an elongate fine pipe electrode having a diameter of 0.1 mm or even less at an improved machining rate.
According to the present invention, a small hole electric discharge machine for forming a small hole in a workpiece by generating electric discharge between a rotating elongate pipe electrode having a diameter of 1.0 mm or less and the workpiece may comprise:
a W-axis slider moveable in the direction of a vertical W-axis;
a Z-axis slider attached to the W-axis slider so as to be moveable in the direction of a vertical Z-axis;
a machine head attached to the Z-axis slider the pipe electrode being attached to the machine head;
a linear motor for moving the Z-axis slider; and
a gas spring for generating a balance force to counteract the gravitational force acting on the Z-axis slider.
The linear motor improves the responsiveness of a machine head to decrease the frequency of short circuits at the work gap and to reduce damage on the pipe electrode due to such short circuits. The gas spring is a balancing device in order to maintain the improved responsiveness.
Additional objects, advantages and novel features of the invention will be set forth in the description that follows, and will become apparent to those skilled in the art upon reading this description or practicing the invention. The objects and advantages of the invention may be realized and attained by practicing the invention as recited in the appended claims.